Field of the Invention
The present application relates generally to hydrotreating processes and more particularly, but not by way of limitation, to improved methods and systems for reactant mixing and distribution in hydrotreating reactors.
History of the Related Art
Hydrotreating refers to a class of catalytic chemical processes for removing impurities such as sulfur, benzene, or the like from petroleum products such as, for example, gasoline, kerosene, diesel fuel, and the like. The purpose of hydrotreating is to reduce emission of pollutants that result from combustion of petroleum products utilized in automotive vehicles, aircraft, railroad locomotives, ships, gas or oil burning power plants, residential or industrial furnaces, and other forms of fuel combustion.
An example of a common hydrotreating reaction involves removal of sulfur from various petroleum products. Such a hydrotreating reaction is referred to as “hydrodesulfurization.” Hydrodesulfurization is of particular importance because sulfur, even in low concentrations, may poison metallic catalysts such as, for example, platinum and rhenium, that are used in refining processes to upgrade an octane rating of petroleum products. Furthermore, sulfur dioxide (SO2) results from combustion of sulfur-contaminated petroleum products. SO2 is a widely-recognized pollutant, which has well-known and wide-ranging detrimental effects on the environment.
Another example of a common hydrotreating reaction involves removal of benzene from petroleum products. In recent years, the Federal government has specified maximum acceptable quantities of benzene that may be present in petroleum products. Thus, refinery operators are required to either capture or destroy benzene present in petroleum products. Capture of benzene represents a substantial capital investment for a refinery. Hydrotreating of benzene is a substantially cheaper alternative and involves reacting a benzene-contaminated petroleum product with hydrogen vapor (H2) in the presence of a catalyst. As a result, benzene is degraded into cyclohexane.
In an industrial hydrotreating unit, such as those found in a refinery, hydrotreating takes place in a reactor column at temperatures ranging from about 300° C. to about 400° C. and pressures ranging from about 30 atmospheres to about 130 atmospheres. Typically, hydrotreating takes place in the presence of a catalyst. Typically, the catalyst is in the form of generally spherical pellets that are packed into various portions of the reactor column known as packed beds.
In operation, reactants descend in a concurrent-flow manner through the reactor column. In most cases, small gaps are present between adjacent catalyst pellets thereby allowing passage of reactants therethrough. The hydrotreating reaction occurs on a surface of the catalyst pellets. In many cases, during operation, reactants are consumed unevenly within the reactor column. Uneven consumption of reactants gives rise to a concentration gradient. The concentration gradient, in many cases, is also accompanied by a temperature gradient across a cross-sectional area of the reactor column. For this reason, reactor columns often include various mixing and distribution devices.
U.S. Pat. No. 7,078,002, assigned to Shell Oil Company, describes a mixing device for mixing fluids in a multiple-bed downflow reactor. The mixing device includes a substantially horizontal collection tray and a swirl chamber arranged below the substantially horizontal collection tray for mixing liquid. The swirl chamber has an upper end part that is in direct fluid communication with an upper surface of the substantially horizontal collection tray and an outlet opening at a lower end. A length of the swirl chamber is at least 0.35 times its inner diameter. The mixing device further includes a substantially horizontal distribution tray beneath the swirl chamber. The substantially horizontal distribution tray includes a plurality of openings for downward flow of liquid and gas.
U.S. Pat. No. 7,052,654, assigned to ExxonMobil Research and Engineering Company, describes a multi-phase mixing system for distributing vapor and liquid across a downflow reactor. The mixing system includes a collection tray for receiving vapor and liquid. The collection tray includes a sub-region. The mixing system further includes a mixing chamber positioned below the collection tray and at least one outlet opening for downward passage of vapor and liquid. The mixing system further includes a conduit extending through the collection tray into the mixing chamber. The conduit permits the flow of vapor and liquid from above the collection tray and into the mixing chamber. The mixing system further includes a vapor slipstream passageway extending through the upwardly projecting sub-region for permitting flow of a vapor slipstream from above the collection tray into the mixing chamber.
U.S. Pat. No. 7,045,103, also assigned to ExxonMobil Research and Engineering Company, describes a distributor system for distributing vapor and liquid across a downflow reactor. The distributor system includes a collection tray for receiving vapor and liquid and a mixing chamber positioned below the collection tray. The mixing chamber has an outlet oriented to permit downward passage of liquid and vapor from the mixing chamber. The distribution system further includes a spillway extending through the collection tray to permit downward passage of vapor and liquid from above the collection tray into the mixing chamber. The distribution system further includes a baffle connected to the collection tray and extending downwardly therefrom into the mixing chamber. The baffle is located between the outlet and the spillway such that a baffle radius is greater than an outlet radius.